This invention relates to a rotary transformer comprising a rotary shaft, a rotor core secured on the rotary shaft, a rotor coil wound about the rotor core, a stator core surrounding the rotor core with an airgap and a stator coil mounted on the stator core, whereby the rotary shaft is rotated to effect transforming with the rotor and stator coils respectively as primary and secondary windings.
Generally a rotary transformer of this type is used as brushless exciter to a thyristor motor or a synchronous motor.
In a conventional rotary transformer of this kind, the rotor core mounted on the rotary shaft includes a cylindrical axial core concentric with the rotary shaft and two radial cores jointed to the opposite ends of the axial core and formed of a number of circular core plates laminated along the axis of the rotary shaft. The stator core of the transformer is constituted by an axial core and radial cores both being formed in the same manner as those of the rotor core. In this device the jointed portions between the cylindrical axial core and the radial cores cause thin air spaces. As a result, a magnetic flux is disturbed at the air gaps whereby its distribution is much confused. To compensate the confusion of magnetic flux distribution, both the rotor and stator cores should have each a coil of an increased number of turns. This means an increase in excitation ampere-turns and a decrease in power-factor of the transformer.
A rotary transformer of another type is known to prevent such a power-factor drop. Its rotor and stator cores are each comprised of a number of U-shaped core plates which are laminated to form a thick cylinder concentrically surrounding the rotary shaft. The transformer is, however, defective in that the attachment of the U-shaped core plates to the rotary shaft to form a rotor core is accompanied with great difficulties, and the core plates can not be so firmly fixed to the shaft as to promise a mechanically strong coreshaft joint.